One of the most notable findings in contemporary behavior genetics is that children growing up in the same family are not very comparable. Findings suggest that in order to understand individual differences between siblings it is necessary to examine not only the shared experiences but also the differences in experiences of children growing up in the same family. In the past decade a group of investigators has begun to examine the contributions of genetics, and both shared and nonshared environment to development. As with many new research endeavors, this has proven to be a difficult task with much controversy and disagreement not only about the most appropriate models and methods of analysis to be used, but also about the interpretation of findings. Written by some of the foremost scholars working in the area on nonshared environment, the papers in this book present their perspectives, concerns, strategies and research findings dealing with the impact of nonshared environment on individual differences in the development of siblings. This volume will have heuristic value in stimulating researchers to think in new ways about the interactions between heredity, shared and nonshared environment and the challenges in identifying their contributions to sibling differences. These papers should raise new questions about how to examine the contributions of genetic and environmental factors to development, with consideration given to the findings of this study of sibling differences and nonshared environment. Further, these papers may encourage a growing trend to integrate genetic and environmental perspectives in studies of development.